This invention is directed generally to repeater systems for wireless communications and more particular to a method and apparatus for improving the isolation between donor and null antennas in a repeater system.
In existing wireless technologies, such as Cellular, PCS (personal communication services), MMDS (multi-user multipath distribution system), WLL (wireless local loop), or the like, repeaters are used to extend coverage of cell sites (base stations) to areas that have low signal reception power, and therefore low signal to noise ratios. One of the principle technology drivers is that of system gain. That is, the higher the gain, the greater the range (distance) and coverage area for the system.
A limiting characteristic for repeaters is that of the feedback loop, or conversely, the isolation between the two opposing antennas (or sensors). That is, the total front to back (F/B) ratio for the system, or isolation, must be higher than the desired gain. Usually, repeaters employ at least one xe2x80x9clinkxe2x80x9d or xe2x80x9cdonorxe2x80x9d antenna which communicates with the base station and at least one broadcast, coverage, or xe2x80x9cnullxe2x80x9d antenna which communicates with the remote stations or subscribers. Generally speaking, the isolation between link and null antennas is equal to the total gain plus some margin, typically around 10 to 15 dB. Therefore, the system gain will in general be less than the isolation minus the margin. For example, if the isolation between antennas is around 60 dB, then the maximum system gain allowed will be about 45 dB. For PCS frequencies, these figures result in a repeater range of less than 100 feet.
In a scattering environment, which is common in PCS, every 6 dB of additional system gain will double the coverage distance. Thus, obtaining an additional 24 dB of isolation between the two antennas, will allow the range to double 4 times, to 1600 feet. For conventional repeater systems, where the two antennas and repeater electronics are in three separate enclosures, and locations, the link (donor) antenna (to the base station) and broadcast (null) antenna (to the desired coverage area), are separated in space by (usually) more than 10 feet. This distance adds over 50 dB to the isolation between antennas, generating a total isolation value of well over 100 dB. Therefore, with a 15 dB margin, this type of system can utilize a total gain of up to 85 dB or more, which results in fairly large range and coverage.
For the side-to-side repeater technologies (as described in the above-referenced prior application), where the opposing antennas are in the same enclosure, and separated in space often by less than a few inches, the F/B ratio (or isolation) is typically limited to a value below 80 dB or so. This therefore allows a total system gain of no more than 65 dB, which limits the system range to a few hundred feet or less.
The invention provides a method and apparatus for improving isolation between donor and null antennas in a repeater system.
The described embodiment of the invention provides an adaptive cancellation approach which removes a significant portion (between 10 dB and 40 dB), of the feedback signal power, therefore increasing the total system isolation by the same amount (10 to 40 dB). This additional isolation can be used to achieve greater system gain, and therefore significantly extend the range of the system. This is especially useful in the side-side repeater technology approach mentioned above and described in the above-referenced application. The cancellation scheme uses digitally processed information to generate a signal, which, when added to the original input signal, cancels the feedback signal.